FIG. 1 depicts electrochemical device 100 having anode 102 and cathode 104. Anode 102 and cathode 104 are separated by separator 106. In one example, electrochemical device 100 is a lithium-ion battery (LIB). Anode 102 includes anode collector 108 and anode material 110 in contact with the anode collector. Cathode 104 includes cathode collector 112 and cathode material 114 in contact with the cathode collector. Electrolyte 116 is in contact with anode material 110 and cathode material 114. Anode collector 108 and cathode collector 112 are electrically coupled via closed external circuit 118. Anode material 110 and cathode material 114 are materials into which, and from which, alkali ions 120 can migrate. During insertion (or intercalation) alkali ions move into the electrode (anode or cathode) material. During extraction (or deintercalation), the reverse process, alkali ions move out of the electrode (anode or cathode) material. When an electrochemical device is discharging, alkali ions are extracted from the anode material and inserted into the cathode material. When the cell is charging, alkali ions are extracted from the cathode material and inserted into the anode material. The arrows in FIG. 1 depict movement of alkali ions through separator 106 during charging and discharging. FIG. 2 depicts electrochemical device 100 positioned in and configured to provide power to apparatus 200. Apparatus 200 may be, for example, a motorized vehicle.
A variety of electrolytes 116 have been used for moving alkali cations from anode to cathode compartments of electrochemical devices. In one example, Li+ ions in a LIB are transported through a molecular solvent blend. The blend is used because no single solvent has been found to dissolve the preferred salt LiPF6 and at the same time yield a sufficiently high ion mobility. Ion mobility can be increased by mixing a high polarity but viscous component with an equal amount of a low dielectric constant, low viscosity, co-solvent. A common electrolyte used in LIBs is LiPF6 dissolved in 1:1 ethylene carbonate-dimethyl carbonate. In some cases, the solution is supported within a gel structure. This electrolyte sacrifices safety (flammability), iconicity, and transport number, but provides acceptable conductivity, and is suitable for use with high voltage cathodes.
A modification of the liquid electrolyte approach that eliminates the molecular solvent, with increase in safety, is the use of ionic liquid solvents for the lithium salt, but this strategy also has the problem that the lithium ion typically becomes the least mobile species in the mixture. This is due to its greater charge intensity that leads it to dominate the electrostatic (or charge polarization) competition for nearest neighbor anions so that it “digs itself a trap”. This problem can typically be mitigated by choosing the least polarizable anions possible, hence the predominance of fluorinated anion species in electrolytes of this type. While cells with such electrolytes can function with high cyclability, the current, hence power, is restricted.
An alternative strategy for avoiding liquid and molecular solvents involves the use of organic cation salts in plastic crystalline states as solvents in which smaller amounts of lithium salts, usually with the same anions, can be dissolved. These electrolytes, however, demonstrate low conductivity and Li+-trapping. Other electrolytes that have been explored include crystalline fast ion conductors like sodium β″ alumina, LiSicon, and thiophosphogermanates, in which the alkali cation is generally the only mobile ion. These electrolytes, however, can have limited appeal based on factors such as toxicity. Moreover, with a few exceptions, their conductivities are typically below 10−2 S/cm at ambient temperature. Fast ion glassy and glass-ceramic electrolytes have also been investigated, but are limited by conductivities that rarely exceed 10−3 S/cm.